In an air drier of the type with two drying towers containing desiccant one of the towers is used for drying air emanating from the compressor in the system, while at the same time the desiccant in the other tower is regenerated by a small fraction of the dried air, which is passed back through the desiccant. The switch-over between the two towers is to be accomplished by a valve, which may also have the function to vent out the regenerating air from the tower that for the time being is not used for drying the air. The valve for these two functions is a 2-way valve with 4 seats and is to be controlled to switch-over between its two positions by an external force.
In the design of the valve the fact that the main air flow from the compressor to be switched-over between the two drying towers is many times bigger than the flow of regenerating air can be taken into account.